Field of the Invention
The present invention relates in general to a sewing machine and in particular to a new and useful method and apparatus for effecting various sewing operations.
A thread monitor for a sewing machine, which has a counter with a counting input, a resetting input, and a presetting input for entering count values is known from U.S. Pat. No. 4,192,243. The pulses of a pulse generator, which generates a predetermined number of pulses, which can be predetermined, per revolution of the main shaft, are sent to the counting input, while a pulse generator driven by the thread pull is connected to the resetting input.
The counter is reset to a starting value by each pulse arriving at the resetting input. Based on this value, the counter sums up the pulses on the counter input until it is again reset by the next resetting pulse. However, when the counter has counted up to the count value set on the presetting input before the next resetting pulse is sent, there is a thread disturbance. Based on this, the counter sends a signal which triggers an alarm or machine stop.
Even though the needle thread can be easily monitored by a thread monitor of such a design to detect thread end and breakage, it is stipulated in that patent that the shuttle thread on a double lock stitch sewing machine can be monitored via the needle thread by detecting its consumption, rather than directly. Therefore, in conjunction with such a sewing machine, this thread monitor is unable to respond to disturbances on the shuttle thread rapidly and with sufficient sensitivity.
British patent specification No. 13 35 677 discloses a sewing machine whose bobbin has a marking formed by light and dark fields on its flange facing a light source and a light receiver. As long as thread is being pulled off, the marking of the rotating bobbin acts as a pulse generator. However, the pulse sequence changes in the case of thread break or thread end, as a result of which the shut-off mechanism of the sewing machine is actuated via electrical or electronic means following the light receiver.
Due to the inertia of the bobbin, thread monitors responding to changes in the pulse sequence have the disadvantage that a reduction of the sewing machine speed brings about bobbin feed. This can cause the bobbin to stop briefly, which causes the thread monitor to respond, despite continued thread pull-off.
U.S. Pat. No. 3,738,296 discloses a bobbin which has a reflective surface. The light beams of a reserve monitoring device, which are deflected by this surface, are sent as light signals to a counter of an external control circuit. The number of bobbin revolutions taking place during sewing is summed up in the counter, and this actual value is compared to a preselected nominal value. The sewing machine is stopped when the two values are equal.
Even though the arrangement described in the U.S. Patent Specification makes it possible to preselect the amount of residual thread according to the needs, rupture of the bobbin thread is not indicated.